In a heatproof bowl over a pot of simmering water (double boiler), melt chocolate with butter until smooth. Remove from heat and let cool slightly.

In a small bowl, whisk egg yolks with hazelnut spread and vanilla until smooth.

In a clean, dry bowl, whisk egg whites with a pinch of salt to soft peaks. Gradually add sugar and continue whipping until glossy stiff peaks form.

In another bowl, whip heavy cream until soft peaks form. Do not overwhip.

Fold the chocolate mixture into the egg yolk mixture gently.

Fold in the whipped cream until partially incorporated.

Finally, fold in the egg whites in batches, being careful to maintain the airy texture.

Spoon the mousse into individual serving cups or glasses.

Cover and refrigerate for at least 2 hours, or until set.

Garnish with toasted hazelnuts, chocolate shavings, cocoa powder, or a dollop of whipped cream. Serve chilled.